Laser beam antenna



Oct. 1, 1968 l.. M. vALLEsE ET AI. 3,404,403

LASER BEAM ANTENNA '.Tiled Jan. 20, 1966 2 Sheets-Sheet 2 a7 y y ,i t 06 /44 4a f R H/GH /A /j/ A A/JfefSz/RE INVENTORS.

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T Amvo/.o snosmk AT TOR NEY United States Patent Ofce 3,404,403 PatentedOct. 1, 1968 3,404,403 LASER BEAM ANTENNA Lucio M. Vallese, Glen Ridge,NJ., and Arnold Shostak,

Arlington- Va., assignors to International Telephone and TelegraphCorporation, Nutley, NJ., a corporation of Maryland Filed Jan. 20, 1966,Ser. No. 521,978 9 Claims. (Cl. 343-700) This invention relates toantennas and more particularly to low frequency (LF) to very lowfrequency (VLF) laser beam antennas.

One of the problems encountered in the construction of known LF to VLFantennas is that very large heights (as high as 400 to 500 meters) mustbe reached by monopole antennas in order that they possess a suitableradiation etliciency. In fact, quarterwave lengths at 500 kc., 100 kc.,50 kc. and 10 kc. are respectively: 150 m., 750 m., 1500 m. and 7500 m.In practice, the construction of such high monopole antennas is veryexpensive, the cost increasing rapidly as a function of the height.

Another disadvantage of known LF to VLF antennas is their need fortowers, thus making them impractical for uses where ease oftransportability is an important factor.

Therefore, it is the main object of this invention to provide a laserbeam antenna which eliminates the need for construction of towers andwhich is highly transportable.

According to this invention a high power laser means for producing atleast one laser beam is coupled to pulsing means for repeatedly pulsingon said laser means. Also coupled to said laser means is focusing meansfor focusing said laser beam at different points to thereby ionize acolumn of air. Means are further provided for coupling a source of inputsignal to the base of said ionized column of air to utilize said columnas an antenna.

The above-mentioned and other objects of this invention will becomeapparent by reference to the following description taken in conjunctionwith the accompanying drawings, in which:

FIGURE l is a block diagram of a laser beam antenna according to thisinvention;

FIGURE 2 is a simplified diagram of apparatus for varying the focallength of a laser beam according to this invention;

FIGURE 3 is a schematic diagram of apparatus for applying an inputsignal to the ionized column of air;

FIGURE 4 is a block diagram of an alternate means for producing a laserbeam antenna utilizing multiple laser beams;

FIGURE 5 is a simplified diagram of apparatus for varying the focallength of a laser beam in a curvilinear manner according to thisinvention; and

FIGURE 6 is a diagram of an auxiliary ionization system for use inconjunction with apparatus according to this invention.

Referring to FIGURE l, a high-power laser (such as a ruby orneodymium-doped laser) 1 is coupled to a pulsing means 3 which pulsessaid laser 1 to the on condition for predetermined lengths of time at apredetermined repetition rate. The laser beam is passed through variablefocusing system 2 and is focused at a predetermined point (point 6, forexample), at which point the air is ionized. A variable focusing system2 is also coupled to pulsing means 3 for causing the focal lengththereof to change in synchronism with the pulsing on of said laser 1.When laser 1 is lirst pulsed on by pulsing means 3, the beam is focused,for example, at point 6 by focusing system 2, ionizing the air at saidpoint 6. During the olf period of said laser 1 pulsing means 3 causesthe focusing system 2 to change its focal length by a predeterminedamount so that during the next on period of laser 1, the beam is focusedat point 7, for example. This process is repeated until a column ofionized air of predetermined height is produced, After the ionizedcolumn is produced once, the focusing system 2 is returned back to itsinitial position to repeat the ionization of the air at all of thesuccessive focal points in order to maintain the ionization of saidcolumn. The repetition rate of the pulsing on of said laser 1 isdetermined by the lifetime of the ions produced at each focal point ofthe laser beam and pulsing means 3 is adjusted accordingly. Also, thefocal points 6, 7 and 8, for example, are spaced such that anapproximately unlform density of ionized plasma is obtained along thecolumn after equilibrium has been reached. Said spacing is achieved byadjustment of focusing means 2 which is described below in more detail.A source of input signals 4 is coupled to the base of said ionizedcolumn via means 5 `to utilize said ionized column as an antenna. Thedetails of these elements are shown more specifically in FIG- URE 3 andare described below with reference thereto.

In FIGURE 2 an embodiment of the variable focusing means 2 of FIGURE lis shown in detail. This is a simplied embodiment and is shown merely byway of example. Lens 10 is mounted in a stationary manner to supports13. Lens 9 is slidably mounted to supports 13 and is further coupled toa rack gear 12, which in turn is coupled to driving gear 11. Saiddriving gear 11 is mechanically coupled to motor 14 which is activatedby said pulsing means 3 via variable timing means 15 and reversing means17 in synchronism with the switching on of laser 1. The setting ofvariable timing means 15 determines how long motor 14 remains on afterit is activated by means 3 and therefore determines by how much thefocal length of focusing system 2 is changed each time said motor 14 isactivated. When motor 14 is activated, lens 9 is caused to move apredetermined distance (determined by the setting of timing means 15) inthe vertical direction by means of gears 11 and 12, thereby changing thefocal length of lens system 9 and 10 by a xed amount. This process issuccessively repeated to provide spaced focal points (points 6, 7 and 8for example) along the length of said ionized column. Counter 16 countsthe number of times motor 14 is activated and after a predeterminednumber of counts, causes the focal length of focusing system 2 to returnto its original value the next time said motor 14 is activated bypulsing means 3. This is accomplished by means of reversing means 17which reverses the direction of rotation of motor 14 when counter 16 hasregistered said predetermined number of counts. Counter 16 also causestimer 15 to allow said motor 14 to be activated for a long enough periodof time to return said focusing system 2 back to its original position.In summary therefore, it is seen that the focal point of the laser beamis caused to change in such a manner as to produce v a column ofsubstantially uniform ionized plasma. When the column reaches apredetermined height, the focal point is returned to its initial valueand again varied in the above manner to maintain said column in anionized condition. It should be noted that the settings of counter 16and variable timer 15 are inter-related. That is, for a given timersetting a fixed number of ionization points (or counter counts) arerequired to produce an ionized column of a given height. If the timedelay of timer 15 is decreased, the spacing between ionization isshortened and therefore a larger number of said ionization points (orcounter counts) are required to produce an ionized column of the sameheight. Therefore, if the setting of timer 15 is varied, the setting ofcounter 16 must be correspondingly varied.

It should be noted that many other methods of changing the focal pointof the laser beam could also be used. For example, in a similar systemto the one of FIGURE 2, both lenses 9 and 10 may be made to movesimultaneously in order to produce equivalent results.

Further, it is possible to utilize focusing devices having no movingparts. For example, the laser beam may be deviated electrically orotherwise so that it is passed successively through different opticalpaths which provide focusing of said beam at the various desired points.Another possibility consists of providing a number of parallel paths forthe laser beam of which only one is opened at a given instant of time.Switching in the latter case, may be implemented by use ofelectro-optical control, magneto-optical control of the polarization,variation of the reectivity of a surface, etc.

FIGURE 3 illustrates a method of exciting an antenna according to thisinvention. This is accomplished by means of transmitter 18 coupled to amatching transformer 19. Transformer 19 is coupled to tuning coil 20which applies the signal from transmitter 18 to the antenna by means ofelectrode 21 immersed in the base of the ionized plasma column. Thematching transformer 19 and tuning coil are similar to types used in theconventional feeding of known types of monopole antenna radiators.Assuming that the column of ionized air is vertical and has height H,the radiation resistance is computed approximately with therelationship:

where )t is the wavelength of the injected RF current and H is assumedto be less than 0.1 The value of the wavelength k must be larger thanthe plasma cutoff wavelength; the latter is determined by the density ofthe plasma and corresponds to an angular frequency:

where N is the ionic density, e the ionic charge, and m the ionic mass.The cutoff value of x is expressed as fol- FIGURE 4 illustrates anotherembodiment of this invention utilizing -two lasers 22 and 23 forproducing an ionized column of air. In this embodiment focusing systems24 and 25 have a curvilinear motion, the detailed operation of which isset forth below with reference to FIG- URE 5. In the circuit of FIGURE 4both lasers are operated by a common pulsing means 26 (similar to means3, FIGURE l), both focusing systems 24 and 25 being operated thereby insubstantially the same manner as in the system of FIGURE 1. Both lasers22 and 23 and both focusing systems 24 and 25 are operated insynchronism and both laser beams are simultaneously successively focusedat the same points in space (points 27, 28, 29 and 30, for example).Utilizing the system of FIGURE 4, lasers having approximately half theoutput power of the laser required in the system of FIGURE l may beutilized to provide results equivalent to the system of FIGURE 1. It isclear that more than two lasers may be combined in a manner similar tothat shown in FIGURE 4 to produce antennas according to this invention.

FIGURE 5 illustrates focusing means 24 and 25 (both are identical) inmore detail. Since they are identical the following discussion willrefer only to focusing means 24. Lens 31 is moved along axis 35 viagears 33 and 34 in the same manner as lens 9 (FIGURE 2) is moved viagears 11 and 12, and further discussion of this movement is deemedunnecessary. Means 36 is further provided to impart curvilinear motionof focusing means 24 about pivot point 37, for example. The angle atwhich the lens system is offset from the vertical axis is represented bya. The curvilinear motion is provided by rack gear 38 which is pivotallycoupled to focusing means 24 at point 41 and which is driven by gear 39which is mechanically coupled to motor 40. Motor is activated in theidentical manner as motor 14 as set forth above in the de- 4 scriptionof FIGURE 2. A successive position of focusing means 24 is shown inFIGURE 5 by the dotted lines. Corresponding elements are denoted bypriming the original designating symbols.

It should also be noted that the ionization phenomenon may be aided byuse of auxiliary means of ionization. One well-known method, shown inFIGURE 6, consists of the use of a high-voltage D.C. or RF discharge ina chamber 42 via source 43 coupled to probe 44 and the simultaneous owof a high-pressure jet of air 45 through aperture 46. Other knownmethods may also be used to aid the laser beam in the ionization of theair, such as the use of high power microwave beams or ultraviolet rays.

The optimization of the selection of the laser for use in this inventionis made on the basis of the peak power output, of the coherence, of thewavelength, etc. For instance, it has been found that, using ruby lasers(wavelength 0.6934 microns), ionization of air at atmospheric pressureis obtained when the laser beam is focused with a peak power density of1015 watts/m.2 or higher, corresponding to an electric field intensityof approximately 10"l volts/ cm. These levels of peak power density andof peak electric field may be obtained by using a pulsed laser havingone joule output energy per pulse and SO-nanosecond pulse duration(i.e., 3() megawatts peak power) focused on a spot of diameter 2 10-2cm. The power required for ionization will increase when the focusingoptical system has optical aberrations. On the other hand, theionization threshold will decrease when the wavelength of the laser isdecreased.

While we have described above the principles of our invention inconnection with specific apparatus, it is to be clearly understood thatthis description is made only by way of example and not as a limitationto the scope of my invention, as set forth in the accompanying claims.

We claim:

1. A laser beam antenna comprising:

laser means for producing at least one laser beam;

means coupled to said laser means for repeatedly pulsing on said lasermeans;

focusing means for focusing said laser beam at different points tothereby ionize a column of air; and

means coupling a source of input signals to the base of said ionizedcolumn of air.

2. An antenna according to claim 1 wherein said focusing means comprisesa variable focal length optical lens system.

3. An antenna according to claim 2 wherein said focusing means furthercomprises means for adjusting the focal length of said focusing means insynchronism with said pulsing on of said laser beam.

4. An antenna according to claim 3 wherein said means for adjustingcomprises:

first ymeans coupled to said pulsing means and to said lens system foradjusting the focal length of said lens system by a predetermined amountresponsive to said pulsing means; and

second means coupled to said rst means for returning the focal lengthback to its starting value after said focal length has been adjusted apredetermined number of times.

5. An antenna according to claim 4 wherein said focusing zmeanscomprises a stationary mounted lens and a slidably mounted lens, saidlenses mounted on a common axis, and wherein said means for adjustingcomprises:

`gear means coupled to said slidably mounted lens for varying theposition of said lens along said axis;

a motor coupled to said gear means;

reversing means coupled to said motor for reversing the rotationthereof;

timing means coupled to said reversing means for permitting said motorto be activated for a predetermined length of time;

-means coupling said timing means to said pulsing means;

and

counting means coupled to said pulsing means, to said timing means andto said reversing means for causing the focal length of said focusingmeans to be returned to its initial value after said laser means hasbeen focused at a predetermined number of points.

6. An antenna according to claim 1 wherein said laser means comprisestwo lasers.

7. An antenna according to claim 6 wherein each said laser is coupled toa focusing means comprising a variable focal length optical lens systemsfor curvilinearly varying said focal points.

8. An antenna according to claim 7 wherein the focal points of each saidlaser are simultaneously adjusted and wherein said focal points are atthe same relative position in space at the same time.

9. An antenna according to claim 8 wherein each said focusing meanscomprises:

mounting means;

a first lens mounted in a stationary manner to said mounting means;

a second lens slidably mounted on said mounting means on a common axiswith said first lens;

first gear 'means coupled to said slidably mounted lens for varying theposition of said lens along said axis; second lgear means coupled tosaid mounting means for rotating the axis of said lenses with respect tothe vertical direction;

a first motor coupled to said first gear means;

a second motor coupled to said second gear means;

reversing means coupled to both said motors;

timing Ameans coupled to said reversing means for permitting said motorsto be activated for a predetermined length of time;

means coupling said timing means to said pulsing means; and

counting means coupled to said pulsing means, to said timing means andto said reversing means for causing the focal points of said laser-means to be returned to their initial positions after said laser beamshave been focused at a predetermined number of successive focal points.

References Cited UNITED STATES PATENTS 1,309,031 7/1919 Hettinger343-701 1,687,792 10/1928 Raue 343-700 2,760,055 8/ 1956 Laster 343-700ELI LIEBERMAN, Primary Examiner.

1. A LASER BEAM ANTENNA COMPRISING: LASER MEANS FOR PRODUCING AT LEASTONE LASER BEAM; MEANS COUPLED TO SAID LASER MEANS FOR REPEATEDLY PULSINGON SAID LASER MEANS; FOCUSING MEANS FOR FOCUSING SAID LASER BEAM ATDIFFERENT POINTS TO THEREBY IONIZE A COLUMN OF AIR; AND MEANS COUPLING ASOURCE OF INPUT SIGNALS TO THE BASE OF SAID IONIZED COLUMN OF AIR.